[Tig] An OLED Story (Ian Richardson)

Richard Kirk richard at filmlight.ltd.uk
Tue Oct 25 09:53:42 BST 2011


Ian Richardson <irichardson at cuttingedge.com.au> sez..

/1. My eyes and my Casio digital still camera see the OLED as slightly magenta, everybody else seems fine.
    Has anybody come across this before?
2. Has anybody had difficulty in measuring across Tube, Plasma, OLED before?
3. Sony recommend a Minolta CA-210 for OLED. Has anybody using one of these measured across Tube, Plasma, and OLED?
4. Has anybody in the group tested with any color meters across Tube, Plasma, OLED and what type of instruments were they?/

Do, please post a follow-up. I am keen to know how this ends. I wonder why we are not seeing this a lot more.

When measuring, trust a well-calibrated spectrometer over the colorimeters. It is very hard to get an accurate set of filters. I find it a permanent struggle to get the red in ours to match for all displays. In the days when all displays were video, and the primary spectra were almost always the same, all colorimeters would measure the same. Even if your filters were random red, green, and blue filters you could correct with a matrix. Now, there are lots of competing technologies with their own primaries, so the red filter has to match the human eye over all wavelengths. You can calibrate a spectrometer for each wavelength band, so they don't have this problem. Human eyes, which have three colour channels and all kinds of other oddities, are down there with the colorimeters, though our brains do an amazing job of hiding this.

I am afraid the Casio camera, your only friend in this story, is probably not entirely trustworthy. Cameras have three channels where many colorimeters have four detectors to get the double peak in the CIE standard observer X. This means that the camera colour spaces have to be fudged so violet and blue appear as different colors. However, it does show that the spectra are significantly different according to some other device with broad RGB detectors.

You clearly have three proper channels to your vision, so color blindness test are unlikely to show up anything. There are genetic variations in the human R and G receptors which can vary our sensitivity peaks by 5nm or so, which would be enough for what you describe. More significantly, there are differences in the yellowness of our eye lenses with age, and the extent of our 'yellow spot' macular dye from person to person. The Truelight "Standard Color Spaces" note lists a number of these sorts of effect.

If you are looking at colors in a darkened room, you will probably need five minutes for your eyesight to adapt. I could imagine you being frog-marched into the theater the moment you came back from holiday and being asked whether the monitors looked right, so I would not trust your first replies. However, I expect you have been starting at them a lot since.

There is a thing called a Nagel anomaloscope that is designed to measure our red-green balance. If you can get one of these (and I haven't been able to get one for myself, yet) then this should be able to tell you whether your eyes are significantly different to everyone else's. If you manage to find an anomaloscope, take care that all measurements are taken at the same temperature (see http://www.nature.com/nature/journal/v363/n6429/full/363546a0.html for an explanation of Richter's original seasonal variation, which was actually the refractive index of the prism changing between winter and summer).

NB:
OLEDs have a nice set of three Gaussian spectral peaks for red, green, and blue. There is nothing wrong with OLEDs - just that they aren't the same as CRT's. Plasmas use the same phosphors as CRTs. LCDs, particularly ones with LED backlights can give anomalous results too.

Cheers.
Richard Kirk

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